Like a dragon cut loose, 18 tons of fire-breathing steel lurched forward for yet one more charge up the Mount Washington Cog Railway. One of the cog stable, Kroflite rattled, shook, belched, and wheezed, pounding home with attitude that the care and feeding of the world's oldest cog railway remains a Sisyphean labor of sooty love.

The summer season for this New England icon opened last weekend with a fleet of seven temperamental steam engines. They appear to have been constructed from leftovers; some are prone to pouting, others just plain stubborn. All the engines, however, celebrate the ingenuity of not only the forefathers who dreamed up a way to mechanically assault a scenic mountain, but also the half-dozen mechanics who continually come up with ways to keep these elderly beasts alive. For some, the effort is passionate.

''Steam is as near as you can get to a living animal that man has created," said Nigel Day, a 47-year-old Welshman. ''These animals breathe, make noises, eat, drink, and throw tantrums. They are steel, alive."

Two of the engines date to the 1870s, and not for the better part of a century has there been a catalog in the world offering new components for the likes of Ammonoosic (No. 2) and Agiocochook (No. 3). Yet on these engines, and all of their younger siblings, piston rings must be replaced every two years, pistons every four years, main axles every five. In fact, nothing on any locomotive is original; all the machines are in a constant state of being rebuilt, then rebuilt again, almost always with parts made or finished on the premises.

The newest engineering plans on file were drafted in 1904; for blueprints the mechanics use accumulated wisdom. But the task is not always a mechanical archeological expedition.

One engine this summer will feed on bio-fuel for the first time in cog history. Designers expect that a diet of vegetable oil mixed with diesel will substantially reduce air pollution and add horsepower. Another engine will be sporting fancy new chrome-plated piston shafts, nitrite-coated axles, and some Teflon-braided plumbing, all in hopes of reducing the wear and tear that comes with constantly climbing a mountain.

No two engines are alike, but they all have shared one trait since the cog's inauguration in 1869 (the year, incidentally, that the Transcontinental Railroad was completed): They are very needy.

The Mount Washington cog has to chug up more than 3 miles of the second steepest track (37.4-percent maximum grade) in the world. Only the Mount Pilatus cog in Lucerne, Switzerland, with a 48 percent grade, is steeper, according to Wayne Presby, a co-owner of the Mount Washington Cog Railway.

Credit for the invention goes to one Sylvester Marsh, who counted Abraham Lincoln as one of his lawyers in a matter unrelated to railroads. Marsh had his eye on Mount Washington when he patented, among other things, a toothed system of wheels that allowed locomotives to creep up and down a central, grooved rail. On Aug. 18, 1868, his concept steamed to the summit -- and has failed, fatally, only twice. For a 50th anniversary celebration in 1929, Old Peppersass was dusted off and fired up, but could not take the strain. The antique skipped off the track and killed its engineer. In 1967, eight passengers died when an engine derailed on a switch that a hiker might have tampered with. Operating procedures and design changes have made a similar tragedy all but impossible, according to railway employees.

No one, however, is promising that a locomotive won't occasionally act up -- maybe blow a piston ring or burst a fitting. Or suddenly announce that it has plain old rattle fatigue.

For the past two winters, the cog has run skiers up a trail groomed on the lower third of Mount Washington, and it was on one of the last days of this year's season that the engine crew invited an observer aboard the cab of Kroflite.

The locomotive is closing in on a half century of service and stands testament to the importance of staying loose -- literally. A bicyclist on metal wheels careering over railroad ties would have some idea of the ride. While the passenger coach offers suspension, the engine offers a jolt with every baby step that the machine's toothed drive wheel takes up the grooves, or rungs, located in the center cog rail.

There are 120,000 rungs (each one needs to be replaced every 20 years) to the top of the mountain. The ski train claws up only about one-third of them. That means 40,000 shocks to the mechanical gut during each 15-minute climb, or 44 jolts per second at a cruising speed of a little better than 3 miles per hour. No wonder these 18-ton engines are needy.

''Sorry 'bout that! Shakes a bit!" Sodergren yelled as Kroflite lurched to life. Sodergren began tweaking valves to keep the boiler pressure at 150 pounds per square inch as the train started up. Crisscrossing the cab floor and moving considerably faster than the mountain scenery was Matt Scott, shoveling coal from the tender into the boiler's maw.

Scott wore sunglasses to protect his eyes from the glare and heat of the fire. Shovel held near the blade, he moved with clockwork precision through a five-second routine. Scoop, turn, jog two steps, flip the latch and open the boiler door, shovel coal in, slam the door shut, latch it, turn, jog two steps, then scoop again . . .

It's bodybuilding in a work environment where earplugs are mandatory, burning bits of coal from the smoke stack fly through the windows to sting like ant bites, and escaping steam from the valves occasionally reduces visibility to a foot or two.

This is the clean part of cog care. The real grit, accumulated since the time Ulysses S. Grant was president, coats the inside of the gym-sized clapboard machine shop located just down the track from the passenger terminal. This is daily home for a coterie of grease-streaked mechanics tending their charges. After Kroflite's round trip, it was where Joe Orlando, 50, the shop foreman, was monitoring the fine-tuning of a steel piston.

He had mounted the 25-pound object on a room-sized Reed Prentice lathe. Forged to rough dimensions outside the shop, the piston was now midway through a four-hour final quarter-inch trim that Orlando directed by constantly adjusting 12 control cranks.

''The thing that impresses me most is not how we keep these locomotives running, but how they did it with cruder tools back then," Orlando explained after removing another 70/1,000ths of an inch of steel. Schooled in his father's machine shop, Orlando has not lost the Brooklyn accent he arrived with in 1987. He had come with his wife to the White Mountains for a vacation and seen a ''machinist wanted" advertisement. The job took, the marriage did not, and now Joseph Orlando III, 16, works alongside his dad.

Mike Kenly was 19 when he started in the shop doing odd jobs. Now 53, he holds the title of chief mechanical officer. There are many fingerprints throughout the cog railway shop. They are on the rail magazines atop the lunch table, they are on the lunch table. They are on coffee mugs, sandwich wrappings, overalls, and everyone's faces. And there is probably not a bolt on a locomotive at the Mount Washington Cog Railway that at one point or another did not have Kenly's fingerprints on it as well.

He has logged 34 years in the shop. What's his secret to job longevity?